Spanish authorities have arrested three men in an operation that has crushed a major botnet network of infected computers.

The Mariposa botnet, which appears to be one of the world's largest, took over millions of computers, many of which continue to be infected, security researchers said: http://www.defintel.com/mariposa.shtml

An informal group of volunteers, calling itself the Mariposa Working Group, disabled Mariposa's command-and-control servers on 23 December and handed over information about the criminals behind it to law enforcement in Spain and the US Spain's Guardia Civil is expected to disclose more details of the arrests today, during a morning press conference.

Mariposa-infected computers were linked to 13 million unique Internet Protocol addresses, said Pedro Bustamante, a researcher with Panda Security. It's hard to pinpoint the exact size of the botnet from that number, but it appears to be one of the world's largest. Researchers studying the notorious Conficker botnet have linked it to half as many IP addresses.

However, with the command-and-control servers in the working group's hands, the infected computers cannot be misused right now.

Researchers have spotted Mariposa infections in half of the Fortune 100, as well as hundreds of government agencies, said Chris Davis, CEO of Defense Intelligence, the company that first identified the botnet in May of last year. Defense Intelligence and Panda Security are part of the Mariposa Working Group, as are researchers at Georgia Tech.

The criminals used Mariposa for typical cyberscams: They stole banking credentials and launched distributed denial-of-service (DDoS) attacks. However, they did not use it to push fake antivirus products, a move that helped keep Mariposa under the radar. "The bot was itself very silent," Bustamante said.

One of those DDoS attacks was directed at Defense Intelligence's computers in Ottawa. Angered by the company's efforts to defeat them, the hackers sent data to the company's servers at the rate of 900M bits per second after they briefly regained control of the botnet on Jan. 25.

Antivirus companies did a good job of detecting some versions of the Mariposa code, but the bad guys changed their software often enough -- sometimes every 48 hours -- that many versions of the malware went undetected. "The AV companies couldn't write signatures fast enough," Davis said.

The researchers say that there are still many Mariposa-infected PCs out there, but they are working with antivirus vendors to improve detection and remove the malicious code from the Internet. Over the next month or two, there should be "a pretty big decline" in the number of infected computers, Davis said.

With the help of the Mariposa Working Group, Spanish police arrested the first Mariposa operator in January in Bilbao, Spain, Davis said. The other two men were arrested last week. He was not authorised to release the names of those arrested.

The criminals' fatal mistake was using a real name while registering command-and-control domains, Davis said. They apparently thought they would be anonymous because they used a private domain name registrar, but that company ended up cooperating with the Mariposa Working Group. "There are so many botnets and so many guys doing this," Davis said, "they get cocky; they get careless."